In the manufacture of paper such as newsprint, one of the finishing operations performed is that of calendaring. One of the variables which is controlled in the course of the calendaring operation to govern the character of the finished product is the thickness of the paper being calendared. One system which is employed to control the web caliper is an arrangement of air showers comprising a plurality of valves which are regulated to control the shower of air directed against the web.
In the prior art, a reel hardness sensor is used to generate a signal which controls the air valves. More particularly, the reel hardness sensor is a disc containing a piezoelectric crystal. The disc rotates against the paper reel to cause the crystal to put out a signal as a measure of reel hardness.
Accurate caliper measurement is found to be of particular importance in newsprint roll building. The system of the prior art described hereinabove does not permit of measurement to afford the desired degree of control of peak to peak profiles as is desirable in newsprint roll building. In addition, in the case of a calendar stack control using reel hardness control, the usage of hot and cold air is excessive owing to the inherent time delay between a corrective calendar stack change and the subsequent result as seen on the reel hardness profile.